A new type of electrical
generator uses bacterial spores to harness the untapped power of evaporating
water, according to research conducted at the Wyss Institute of Biologically
Inspired Engineering at Harvard University. Its developers foresee electrical
generators driven by changes in humidity from sun-warmed ponds and harbors.

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The prototype generators work by
harnessing the movement of a sheet of rubber coated on one side with spores.
The sheet bends when it dries out, much as a pinecone opens as it dries or a
freshly fallen leaf curls, and then straightens when humidity rises. Such
bending back and forth means that spore-coated sheets or tiny planks can act as
actuators that drive movement, and that movement can be harvested to generate
electricity.

"If this technology is
developed fully, it has a very promising endgame," said Ozgur Sahin,
Ph.D., who led the study, first at Harvard's Rowland Institute, later at the
Wyss Institute, and most recently at Columbia University, where he's now an
associate professor of biological sciences and physics. Sahin collaborated with
Wyss Institute faculty L. Mahadevan, Ph.D., and Loyola University faculty Adam
Driks, Ph.D. The researchers reported their work in late January in Nature
Nanotechnology.

Water evaporation is the largest
power source in nature, Sahin said. "Sunlight hits the ocean, heats it up,
and energy has to leave the ocean through evaporation," he explained.
"If you think about all the ice on top of Mt. Everest — who took this huge
amount of material up there? There's energy in evaporation, but it's so subtle
we don't see it."

But until now no one has tapped
that energy to generate electricity.

As Sahin pursued the idea of a
new humidity-driven generator, he realized that Mahadevan had been
investigating similar problems from a physical perspective. Specifically, he
had characterized how moisture deforms materials, including biological
materials such as pinecones, leaves and flowers, as well as man-made materials
such as a sheet of tissue paper lying in a dish of water.

Sahin collaborated with Mahadevan
and Driks on one of those studies. A soil bacterium called Bacillus
subtilis, wrinkles as it dries out like a grape becoming a raisin, forming
a tough, dormant spore. The results, which they reported in 2012 in the Journal
of the Royal Society Interface, explained why.

Unlike raisins, which cannot
re-form into grapes, spores can take on water and almost immediately restore
themselves to their original shape. Sahin realized that since they shrink
reversibly, they had to be storing energy. In fact, spores would be
particularly good at storing energy because they are rigid, yet still expand
and contract a great deal, the researchers predicted.